I did the whole replacing RIFF thing, but every single time I started up the game it would crash (Although that might be because of file size).Yes, that would be a bad idea.
All right, I took another look.
You will notice at the start of the file after X_DAMAG1.WAV you will find the hex codes 00 1D 2B 00. Before that you will find E1 00 00 00. If you go to http://www.binaryhexconverter.com/hex-to-decimal-converter (or use whatever hex-to-dec converter you have handy) and convert "2B 1D" (note that the order is reversed) and "E1", you get 11037 and 225. Your hex editor will tell you that the first occurrence of "RIFF" appears at byte 225 and the second occurrence is at 11037.
So, if you want to insert a longer file, you'll have to insert the new data, delete the old data, and then find the new locations of each occurrence of "RIFF" and then update the appropriate hex codes that appear between the file names at the start of the file. (Remember to get the order correct!) The last number before the first occurrence of "RIFF" is "12 41 03"; this appears to be the total size of the file plus one. (Covert "03 41 12" and you get 213266.) You'll need to update that too.
Alternatively, if you already have all your WAV files, you can paste them together on the Windows command line by using
copy /b file1.wav + file2.wav + file3.wav VO_PL00.SEZ
Then you can copy the first 225 bytes of the original VO_PL00.SEZ (everything before the first occurrence of "RIFF") and insert it at the start of your new VO_PL00.SEZ. Then find the locations of each occurrence of "RIFF" and update the hex codes.
This is about as easy as it ever gets.
Also, your WAV files probably have to be uncompressed 8-bit 22050 Hz Mono.